[time-nuts] time-nuts Digest, Vol 95, Issue 74

[John Lofgren]

Yes, Adam, you're correct.  In RF board layouts it's usually good practice to not allow ground fill under most components on the copper layer where the component is mounted.  Some specific exceptions exist for filters, amplifiers, etc. where the copper is needed for lowering the ground impedance or increasing thermal flow.

In the specific case with the crystal filters the ground was also opened up in the layers below the inductors.  It was either a 4 or 6 layer board and we made a rectangular cutout in the fill on the RF ground layer and power layer below each inductor.  The bottom layer ground fill was left intact.

Part of the issue with the board we were updating was that the RF ground layer was very close to the top component layer.  The whole board thickness was something less than the usual 0.062" and the height from the top to RF ground was something like 0.008".  In this case there's pretty strong coupling from the inductors into the ground.  On a 2 sided 0.062" board it's probably less of an issue.

The second part of the issue was the inductor type.  These were wirewound inductors with values in the mH range.  Their external field (and stray coupling) will be different than a nH range ceramic multilayer part.

As always, YMMV.  Each case will be a little different depending on frequency, component construction, and board stack.

-John


-----Original Message-----
From: Adam [mailto:abkhi at gmx.com] 
Sent: Friday, June 22, 2012 10:04 AM
To: time-nuts at febo.com
Cc: John Lofgren
Subject: Re: time-nuts Digest, Vol 95, Issue 74

I'm learning a bunch here about filters and pcb techniques. Got one
question that I'm hoping somebody could help me out with:
> Date: Thu, 21 Jun 2012 13:39:01 +0000
> From: John Lofgren <jlofgren at lsr.com>
>
> One thing we have found that helps, though, is voiding the ground underneath the inductor. 
I did some searching on Google, and found the term "voiding the ground"
with regard to pcb's, but I couldn't find anywhere that actually said
what that meant. Is it getting rid of the copper under the component?
Does that apply to both sides of a double sided pcb? Anybody willing to
help a newbie?

Inquiring minds . . . .

thanks,

--adam, wh6m